<?xml version="1.0" encoding="utf-8" ?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
<!-- Copyright Aleksey Gurtovoy 2006. Distributed under the Boost -->
<!-- Software License, Version 1.0. (See accompanying -->
<!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<meta name="generator" content="Docutils 0.3.6: http://docutils.sourceforge.net/" />
<title>THE BOOST MPL LIBRARY: ETI</title>
<link rel="stylesheet" href="../style.css" type="text/css" />
</head>
<body class="docframe">
<table class="header"><tr class="header"><td class="header-group navigation-bar"><span class="navigation-group"><a href="./incomplete-support-for.html" class="navigation-link">Prev</a>&nbsp;<a href="./resources.html" class="navigation-link">Next</a></span><span class="navigation-group-separator">&nbsp;|&nbsp;</span><span class="navigation-group"><a href="./incomplete-support-for.html" class="navigation-link">Back</a>&nbsp;Along</span><span class="navigation-group-separator">&nbsp;|&nbsp;</span><span class="navigation-group"><a href="./portability.html" class="navigation-link">Up</a>&nbsp;<a href="../index.html" class="navigation-link">Home</a></span><span class="navigation-group-separator">&nbsp;|&nbsp;</span><span class="navigation-group"><a href="./tutorial_toc.html" class="navigation-link">Full TOC</a></span></td>
<td class="header-group page-location"><a href="../index.html" class="navigation-link">Front Page</a> / <a href="./technical-details.html" class="navigation-link">Technical Details</a> / <a href="./portability.html" class="navigation-link">Portability</a> / <a href="./eti.html" class="navigation-link">ETI</a></td>
</tr></table><div class="header-separator"></div>
<div class="section" id="eti">
<h1><a class="toc-backref" href="./portability.html#id76" name="eti">ETI</a></h1>
<p>In context of C++ template problems, ETI is an abbreviation for &quot;Early
Template Instantiation&quot; — a Microsoft Visual C++ - specific issue that 
has been a barrier to any serious work with templates on this platform until 
Microsoft developers fixed it in Visual C++ 7.1 (2003 .NET). Although the
problem is relatively easy to work around if the right techniques 
are applied systematically through the codebase, the approach is definitely
tedious and time-consuming. So, if one day you discover that you are spending 
too much time dealing with the issue, consider upgrading to the
newer version of the compiler. In fact, seriously consider it regardless.
The benefits of saved time, money and frustration are well worth the price.</p>
<div class="section" id="eti-the-problem">
<h2><a name="eti-the-problem">The Problem</a></h2>
<p>Here is a short demonstration of the issue with MSVC 6.x:</p>
<pre class="literal-block">
template&lt; typename F, typename T &gt; struct apply1
{
    typedef typename F::template apply&lt;T&gt;::type type;
};
</pre>
<p>Trying to compiling this innocent-looking code, we get:</p>
<pre class="literal-block">
portability.cpp(4) : error C2903: 'apply' : symbol is neither a class template 
            nor a function template
        portability.cpp(5) : see reference to class template instantiation 
            'apply1&lt;F,T&gt;' being compiled
portability.cpp(4) : error C2143: syntax error : missing ',' before '&lt;'
        portability.cpp(5) : see reference to class template instantiation 
            'apply1&lt;F,T&gt;' being compiled
portability.cpp(4) : error C2059: syntax error : '&lt;'
        portability.cpp(5) : see reference to class template instantiation 
            'apply1&lt;F,T&gt;' being compiled
</pre>
<p>The &quot;symbol is neither a class template nor a function template&quot; part of the 
diagnostics is actually often an indication of ETI-related problems. Another 
typical error message usually says something about nested type such-and-such 
not being a member of a global namespace.</p>
<p>Both cases are two sides of the same compiler bug, which we call
&quot;Early template instantiation&quot;: the compiler, for internal
purposes, in order to process class template definitions,
instantiates class templates with dummy template parameters
(<tt class="literal"><span class="pre">int</span></tt>'s). That can happen both during parsing of template
definitions (and such errors are most easy to identify and fix —
the template definition itself just doesn't compile; the example
above falls into this category), or later during template
instantiation, and these one are hard to detect — the bug will
only be triggered in some particular context.</p>
<!-- namespace-scope: nested templates are immune? -->
<p>ETI is always performed during parsing of the namespace-scope
template definition, which basically means that any template
definition that is rendered invalid by substituting its template
parameters by <tt class="literal"><span class="pre">int</span></tt>s might not compile, as it happened with our
example:</p>
<pre class="literal-block">
template&lt; typename F, typename T &gt; struct apply1
{
    // typedef typename F::template apply&lt;T&gt;::type type;
    // ETI generates this: 
    typedef typename int::template apply&lt;int&gt;::type type;
};
</pre>
<p>If you compile this, you'll get <em>exactly</em> the same diagnostics as we've just seen.</p>
<p>Note that we've said &quot;might not compile&quot;, because... well, the short answer is, 
&quot;it depends&quot;. We haven't analyzed things to the point that we could tell you the 
exact condition when ETI leads to an error and when it doesn't, but
that's not very important anyway — if it's an error, you just fix it (we'll show you how
in a second), and if it's not, then you leave things as is. If one day the 
potential issue turns into a real one, then you apply the workaround we are about
to give you.</p>
</div>
<div class="section" id="eti-the-symptoms">
<h2><a name="eti-the-symptoms">The Symptoms</a></h2>
<p>We've already looked at the typical diagnostics, so we won't repeat
ourselves.  Instead we'll
just mention that many MSVC's INTERNAL COMPILER ERRORs (ICEs) are, in fact, caused
by an ETI-related problem somewhere deep down the instantiation stack.</p>
</div>
<div class="section" id="eti-the-solution">
<h2><a name="eti-the-solution">The Solution</a></h2>
<p>There is no way we can change the compiler's behavior in this case, so what we have
to do is to adjust to it and still make our templates do what we want. Surprisingly,
in most cases it's quite simple to achieve:</p>
<pre class="literal-block">
// potentially unsafe
template&lt; typename F &gt; struct apply0
{
    typedef typename F::type type;
};

// now ETI-safe
template&lt;&gt; struct apply0&lt;int&gt;
{
    typedef int type;
};
</pre>
<p>Since the original template could have never been instantiated with <tt class="literal"><span class="pre">int</span></tt>,
providing a stub <tt class="literal"><span class="pre">int</span></tt> specialization is completely innocent.</p>
<!-- Looks like you're missing lots of stuff, like ETI_BASE - - no? -->
</div>
</div>

<div class="footer-separator"></div>
<table class="footer"><tr class="footer"><td class="header-group navigation-bar"><span class="navigation-group"><a href="./incomplete-support-for.html" class="navigation-link">Prev</a>&nbsp;<a href="./resources.html" class="navigation-link">Next</a></span><span class="navigation-group-separator">&nbsp;|&nbsp;</span><span class="navigation-group"><a href="./incomplete-support-for.html" class="navigation-link">Back</a>&nbsp;Along</span><span class="navigation-group-separator">&nbsp;|&nbsp;</span><span class="navigation-group"><a href="./portability.html" class="navigation-link">Up</a>&nbsp;<a href="../index.html" class="navigation-link">Home</a></span><span class="navigation-group-separator">&nbsp;|&nbsp;</span><span class="navigation-group"><a href="./tutorial_toc.html" class="navigation-link">Full TOC</a></span></td>
</tr></table></body>
</html>
